Delay diversity in antenna arrays

ABSTRACT

In a wireless transmission system including a transmit delay module, delaying each of at least one copy of a signal by a respective delay, the signal being transmitted over a plurality of carrier frequencies and over at least one beam, the at least one beam exhibiting a beam pattern, the number of copies of the signal corresponding to the number of beams, each the at least one copy being associated with a respective one of the at least one beam, the system further including a beam pattern former, coupled with the transmit delay module, producing a plurality of transmit signals corresponding to the beam pattern, a frequency dependent beam shifter, coupled with the beam pattern former, delaying each of the at least one transmit signal by a respective angular shift delay, thereby applying an angular shift to each of the at least one beam, the angular shift of each of the at least one beam corresponding to at least a respective one of the carrier frequencies, wherein each beam is transmitted with a delay corresponding to the transmit delay of each the copy.

FIELD OF THE DISCLOSED TECHNIQUE

The disclosed technique relates to wireless communications, in general,and to methods and systems for employing delay diversity in antennaarrays, in particular.

BACKGROUND OF THE DISCLOSED TECHNIQUE

Communication channels (e.g., wireless, wired and optical), especiallywireless channels, exhibit noise which decreases the reliability of thereceived signal (i.e., the transmit signal is not correctly received atthe receiver). Techniques known in the art to increase the reliabilityof the received signal include Forward Error Corrections (FEC),equalization and transmission diversity. Transmission diversity includestime diversity, delay diversity, frequency diversity and spacediversity. In space diversity, copies of the transmit signal propagatedvia different paths toward the receiver. The receiver combines thesecopies to increase the received signal power. In frequency diversity,copies of the information signal are modulated over a number ofdifferent carrier frequencies. The receiver receives each of themodulated carrier frequencies and combines the received signals.According to the delay diversity technique, a transmitter transmits thesame signal several times, each time at a different time-delay. When atransmitter employs a single antenna the antenna transmits the signalover an omni-directional beam of an electromagnetic wave and thetransmitter transmits delayed versions of the signal via the singleantenna. The transmitter may employ a plurality of antennas (i.e., anarray of antennas), and transmit the signal via the antennas, at atime-delay associated with each antenna. When the transmitter transmitsthe signal via the antennas, at the time-delay associated with eachantenna, and the transmissions of the delayed signal overlap, thefrequency response of the communication channel (i.e., the attenuationand the phase shift of the channel caused by interference in the channelat different frequencies) may attenuate at certain frequencies where thedelayed transmitted signals destructively interfere with each other(i.e., the channel is a frequency selective channel). Furthermore, thetime-delay between the transmitted signals (i.e., the signalstransmitted by each of the antennas) introduces a phase-shift betweenthe transmitted signals. Thus, instead of an omni-directional beam of anelectromagnetic wave, created when a single antenna is used, theantennas create beams which exhibit spatial directionality. Thisdirectionality is a result of the destructive and constructiveinterference of the transmitted signals in space (i.e., similar to adiffraction pattern of a plurality of point light sources). In general,the maximum number of beams produced corresponds to the number ofantennas. The number of beams together with the direction, width andlength of the beams is referred to herein as the ‘beam pattern’. Due tothe spatial directionality of the beams, two receivers for example,located at two different spatial locations relative to the transmittingantennas, may receive the transmitted signal at different receivedlevels of power.

As mentioned above, the direction of the beams is determined accordingto the relative time-delay or, alternatively, the relative phase-shiftbetween the transmitted signals.

Reference is now made to FIGS. 1A, 1B and 1C. FIGS. 1A and 1B areschematic illustrations of two signals 10 and 12 respectively, with atime difference ΔT there between. FIG. 1C is a schematic illustration ofan exemplary transmitter, generally referenced 20, for transmitting asignal using delay diversity. Transmitter 20 includes two antennas 22and 24 and a beam former 30. Beam former 30 includes two delays 26 and28. Antenna 22 is coupled with delay 26 and antenna 24 is coupled withdelay 28. A signal X is provided to delay 26 and to delay 28. Delay 26delays signal X by T₀ (e.g., T₀ is equal to zero in FIG. 1A). Delay 28delays signal X by T₁ wherein T₁−T₀=ΔT (e.g., T₁ is equal to ΔT in FIG.1B). Antenna 22 transmits the signal delayed by T₀ and antenna 24transmits the signal delayed by T₁. As a result of delays introduced tosignal X by delays 26 and 28 of beam former 30, the signals transmittedby each antenna undergo constructive and destructive interference.Consequently, system 20 transmits the signal over a beam 32 ofelectromagnetic waves, where beam 32 exhibits spatial directionality.The direction of beam 32 is determined according to ΔT and thetransmitted frequency. Furthermore, beam 32 may exhibit attenuation atcertain transmitted frequencies. It is noted that, in fact, system 20produces two beams, directed in opposite directions however, only onebeam is depicted in FIG. 1C.

U.S. Patent application publication 2006/0168165, to van Nee, entitled“Delay Diversity and Spatial Rotation Systems and Methods” is directedtowards a system and a method for combining delay diversity and spatialrotation. The system directed to by van Nee includes a Forward ErrorCorrection (FEC) encoder, a puncture module, a spatial stream parser, aplurality of interleavers and a plurality of modulators. The systemdisclosed by van Nee further includes a cyclic delay module, a Walshmatrix operator, a plurality of Inverse Fast Fourier Transform (IFFT)modules, a plurality of RF/analog modules and a plurality of antennas.Each antenna is coupled with a respective RF/analog module. Each IFFTmodule is coupled with a respective RF/analog module and with the Walshmatrix operator. The cyclic delay module is coupled with the Walshmatrix operator and with each of the modulators. Each frequencyinterleaver is coupled with a respective modulator and with the spatialstream parser. The puncture module is coupled with the spatial streamparser and with the FEC encoder.

An input data stream is provided to the FEC encoder which encodes theinput data stream to create codewords. The puncture module removesredundant bits from the encoded data stream. The spatial stream parserseparates the input data stream into a number of spatial streams. Eachfrequency interleaver re-orders the bits of the spatial streams suchthat the transmitted spatial streams are not mirror images of eachother. Each modulator modulates the interleaved spatial stream providedby the respective frequency interleaver coupled thereto.

The cyclic delay module introduces to each spatial stream a cyclicdelay. The output of the delay modules are cyclically delayed spatialstreams. The number of cyclically delayed spatial streams may bedifferent from the number of spatial streams at the input of the cyclicdelay module. The Walsh matrix operator introduces a spatial rotationfor each cyclically delayed spatial stream thereby mapping each delayedspatial stream to a transmit signal. The IFFT modules combine spatialstreams and the sub-carriers into time-domain signals which are used bythe RF/analog modules for transmissions by the antennas.

SUMMARY OF THE PRESENT DISCLOSED TECHNIQUE

It is an object of the disclosed technique to provide a novel method andsystem for employing delay diversity in antenna arrays. In accordancewith the disclosed technique, in a wireless transmission system whichincludes a transmit delay module coupled with a beam pattern formerthere is thus provided a frequency dependent beam shifter, coupled withthe beam pattern former. The transmit delay module delays each of atleast one copy of a signal by a respective delay. The signal istransmitted over a plurality of carrier frequencies and over at leastone beam. The at least one beam exhibits a beam pattern. The number ofcopies of the signal corresponds to the number of beams. Each of the atleast one copy is associated with a respective one of the at least onebeam. The beam pattern former produces a plurality of transmit signalscorresponding to the beam pattern. The frequency dependent beam shifterdelays each of the at least one transmit signal by a respective angularshift delay. Thereby, the frequency dependent beam shifter applies anangular shift to each of the at least one beam. The angular shift ofeach of the at least one beam corresponds to at least a respective oneof the carrier frequencies. Each beam is transmitted with a delaycorresponding to the transmit delay of each copy.

In accordance with another aspect of the disclosed technique, there isthus provided a wireless transmission system. The wireless transmissionsystem includes a transmit delay module, a beam pattern former and afrequency dependent beam shifter. The beam pattern former is coupledwith the transmit delay module and with the frequency dependent beamshifter. The transmit delay module delays each of at least one copy of asignal by a respective delay. The signal is transmitted over a pluralityof carrier frequencies and over at least one beam. The at least one beamexhibits a beam pattern. The number of copies of the signal correspondsto the number of beams. Each the at least one copy is associated with arespective one of the at least one beam. The beam pattern formerproduces a plurality of transmit signals corresponding to the beampattern. The frequency dependent beam shifter delays each of thetransmit signals by a respective angular shift delay. Thereby, thefrequency dependent beam shifter applies an angular shift to each of theat least one beam. The angular shift of each of the at least one beamcorresponds to at least a respective one of the carrier frequencies.Each beam is transmitted with a delay corresponding to the transmitdelay of each copy.

In accordance with a further aspect of the disclosed technique, there isthus provided a method for transmitting a signal over a plurality ofcarrier frequencies and a plurality of beams. The beams exhibits a beampattern, the method comprising the procedures of delaying each copy ofthe signal by a corresponding transmit delay and applying a beam formingmatrix to the delayed copies of the signal, thereby producing transmitsignals corresponding to the beam pattern. Each transmit signal beingassociated with at least one carrier frequency. The method furtherincludes the procedure of delaying each of the transmit signals by arespective angular shift delay, thereby applying an angular shift toeach beam in the beam pattern. The angular shift of each beamcorresponds to each carrier frequency.

BRIEF DESCRIPTION OF THE DRAWINGS

The disclosed technique will be understood and appreciated more fullyfrom the following detailed description taken in conjunction with thedrawings in which:

FIGS. 1A and 1B are schematic illustrations of two signals with a timedifference there between as known in the art;

FIG. 1C is a schematic illustration of an exemplary transmitter fortransmitting a signal using delay diversity as known in the art;

FIGS. 2A, 2B, 2C and 2D are schematic illustrations of a system,constructed and operative in accordance with an embodiment of thedisclosed technique;

FIG. 3 is a schematic illustration of a system for producing beam delaydiversity with frequency dependent beam direction shifting, constructedand operative in accordance with another embodiment of the disclosedtechnique;

FIG. 4A, is a schematic illustration of an exemplary system forproducing beam delay diversity with frequency dependent beam directionshifting, constructed and operative in accordance with a furtherembodiment of the disclosed technique;

FIG. 4B, which is a schematic illustration of a system for employingspatial delay diversity with carrier frequency dependent beam directionshifting, constructed and operative in accordance with anotherembodiment of the disclosed technique;

FIG. 5 is a schematic illustration of a system for employing spatialdelay diversity with frequency dependent beam direction shifting,constructed and operative in accordance with a further embodiment of thedisclosed technique;

FIG. 6 is a schematic illustration of a system for employing spatialdelay diversity and frequency dependent beam direction shifting,constructed and operative in accordance with another embodiment of thedisclosed technique;

FIG. 7 which is a schematic illustration of a system for producingtransmit delay diversity with frequency dependent beam directionshifting, operative in accordance with a further embodiment of thedisclosed technique; and

FIG. 8 which is a schematic illustration of a method for producing beamdelay diversity with frequency dependent beam direction shifting,operative in accordance with another embodiment of the disclosedtechnique;

DETAILED DESCRIPTION OF THE EMBODIMENTS

The disclosed technique overcomes the disadvantages of the prior art byproviding a system and a method for introducing delay diversity intransmitters employing antenna arrays which transmit according towide-band multi-carrier transmission schemes (e.g., Wi-Fi, WCDMA, UMTS)while reducing channel selectivity (i.e., reducing the frequency bandswhereat the channel attenuates the transmitted signal). In multi-carriertransmission schemes, the transmission bandwidth includes a plurality ofnarrow band sub-carriers. The sub-carriers are modulated by themodulating symbols intended for transmission (e.g., synchronizationsymbols, data symbols). The term ‘signal’ refers herein to anelectromagnetic signal transmitted by a transmitter (e.g., asynchronization signal, a data signal). In multi-carrier transmissionschemes, the signal is an aggregation of the plurality of thesub-carriers. The parameters characterizing multi-carrier transmissionschemes include two time parameters. The first time parameter is thesymbol period (i.e., determined according to the Fourier transforminterval of the multi-carrier transmission scheme), which is inverselyproportional to the sub-carrier bandwidth. The second time parameter isthe signal sample period (i.e., determined according to the minimumNyquist sampling rate), which is inversely proportional to the bandwidthof the signal.

The system according to the disclosed technique transmits copies of thesignal over a plurality of beams. A beam former determines the spatialdirectionality of each beam. Thus, each beam exhibits correspondingspatial directionality. A transmit delay module delays each copy of thesignal prior to it being provided to the beam former. Thus, each beam isassociated with a respective time delay. Furthermore, a frequencydependent beam shifter delays the signals produced by the beam patternformer. This delay, referred to herein as ‘angular shift delay’,produces an angular shift in the direction of each beam. This angularshift depends on the sub-carrier frequency. In other words, thedirection of each beam changes according to the sub-carrier frequency.Thus, each beam is directed toward a corresponding direction at arespective time delay. This direction is associated with a respectivecarrier frequency. Thus, the average transmission power of the signal isisotropic. Both of the above mentioned delays may be replaced with aphase shifter. Furthermore, the direction corresponding to each beam maybe essentially confined in a determined angular sector.

Reference is now made to FIGS. 2A, 2B, 2C and 2D which are schematicillustrations of a system, generally referenced 100, constructed andoperative in accordance with an embodiment of the disclosed technique.System 100 produces beam delay diversity with frequency dependent beamdirection shifting, and is operative in accordance with a furtherembodiment of the disclosed technique. System 100 includes a transmitter102 coupled with antennas 104 ₁, 104 ₂ and 104 _(M). System 100transmits a signal X, for example, over four directional beams 106, 108,110 and 112 using three sub-carrier frequencies F₁, F₂ and F₃. Signal Xincludes a plurality of symbols, each associated with the same symbolperiod. Referring to FIG. 2A, transmitter 102 transmits a signal X witha first sub-carrier frequency F₁. Transmitter 102 produces beams 106,108, 110 and 112. Each of beams 106, 108, 110 and 112 is associated witha respective transmit delay. Beam 106 is associated with a transmitdelay T₁. Beam 108 is associates with a transmit delay T₂. Beam 110 isassociates with a transmit delay T₃ and beam 112 is associated with atransmit delay T₄. Furthermore each of beams 106, 108, 110 and 112 isdirected towards a corresponding different spatial direction. Thedirection corresponding to each of beams 106, 108, 110 and 112 isassociated with sub-carrier frequency F₁. Accordingly, Beam 106 isdirected at a direction indicated by an arrow 114. Beam 108 is directedat a direction indicated by an arrow 116. Beam 110 is directed at adirection indicated by an arrow 118. Beam 112 is directed at a directionindicated by an arrow 120. Thus, user 122 receives signal X over beam106 with delay T₁ and user 124 receives signal X over beam 108 withdelay T₂.

Referring to FIG. 2B, transmitter 102 transmits the signal X with asecond sub-carrier frequency F₂. Each of beams 106′, 108′, 110′ and 112′is associated with the respective transmit delay. Beam 106′ isassociated with transmit delay T₁. Beam 108′ is associates with transmitdelay T₂. Beam 110′ is associates with transmit delay T₃ and beam 112′is associated with transmit delay T₄ Each of beams 106′, 108′, 110′ and112′ is directed towards a corresponding spatial direction. Thedirection corresponding to each of beams 106′, 108′, 110′ and 112′ isassociated with sub-carrier frequency F_(2.) The direction correspondingto each of beams 106′, 108′, 110′ and 112′, respective of frequency F₂exhibits an angular shift relative to the direction corresponding toeach of beams 106, 108, 110 and 112 respective of frequency F₁ (i.e., asdepicted in FIG. 2A). Accordingly at sub-carrier frequency F₂, beam 106′is directed toward a direction indicated by an arrow 126. Beam 108′ isdirected toward a direction indicated by an arrow 128. Beam 110′ isdirected toward a direction indicated by an arrow 130. Beam 112′ isdirected toward the directions indicated by arrows 132 and 134.Accordingly, users 122 and 124 receive signal X from beam 106 with delayT₁. It is noted than beam 112 is divided into two parts due to thecyclic nature of the signal

Referring to FIG. 2C, transmitter 102 transmits the signal X with athird sub-carrier frequency F₃. Each of beams 106″, 108″, 110″ and 112″is associated with a respective transmit delay. Beam 106″ is associatedwith transmit delay T1. Beam 108″ is associates with transmit delay T₂.Beam 110″ is associated with transmit delay T₃ and beam 112″ isassociated with transmit delay T₄ Each of beams 106″, 108″, 110″ and112″ is directed towards a corresponding spatial direction. Thedirection corresponding to each of beams 106″, 108″, 110″ and 112″ isassociated with sub-carrier frequency F₃ The direction corresponding toeach of beams 106″, 108″, 110″ and 112″, respective of frequency F₃exhibits an angular shift relative to the direction corresponding toeach of beams 106, 108, 110 and 112 respective of frequency F₁ (i.e., asdepicted in FIG. 2A). Furthermore, direction corresponding to each ofbeams 106″, 108″, 110″ and 112″ exhibits an angular shift relative tothe direction corresponding to each of beams 106′, 108′, 110′ and 112′respective of frequency F₂ (i.e., as depicted in and 2B). Accordingly,at sub-carrier frequency F₃, beam 106″ is directed at a directionindicated by an arrow 134. Beam 108″ is directed at a directionindicated by an arrow 136. Beam 110″ is directed at a directionindicated by an arrow 138. Beam 112″ is directed at a directionindicated by an arrow 140. Accordingly, user 122 receives signal X overbeam 110 with delay T₂ and user 124 receives signal X over beam 106 withdelay T₁.

Referring to FIG. 2D, beams 106′, 108′, 110′ and 112′ respective offrequency F₂ are depicted overlaid on beams 106, 108, 110 and 112respective of frequency F₁. As can be noted from the figure, beams 106′,108′, 110′ and 112′ exhibit an angular shift in the direction there ofrelative to beams 06, 108, 110 and 112.

As mentioned above, the beam delay diversity with frequency dependentbeam direction shifting, described herein above in conjunction withFIGS. 2A, 2B, 2C and 2D, can be achieved by introducing time delaysbefore and after the beam former. Reference is now made to FIG. 3, whichis a schematic illustration of a system, generally referenced 200, forproducing beam delay diversity with frequency dependent beam directionshifting, constructed and operative in accordance with anotherembodiment of the disclosed technique. System 200 includes a transmitdelay module 202, a beam pattern former 206, an beam frequency dependentshift former 210, a multi-carrier modulator 212, a front end interface216 and a plurality of antennas 218 ₁-218 _(M). In system 200, transmitdelay module 202 includes a plurality of first delay modules 204 ₁-204_(N) where N is the number of beams. Beam frequency dependent shiftformer 210 includes a plurality of angular shift delay modules 208 ₁-208_(M) (i.e., the number of angular shift delay modules is equal to thenumber of antennas). Multi-carrier modulator includes a plurality ofmodulators 214 ₁-214 _(M) each associated with a respective sub-carrierfrequency. Beam pattern former 206 is coupled with each of first delaymodules 204 ₁-204 _(N) and with each of angular shift delay modules 208₁-208 _(M). Front end interface 216 is coupled with each ofmulti-carrier modulators 214 ₁-214 _(M) and with each of antennas 218₁-218 _(M). Each of angular shift delay modules 208 ₁-208 _(M) isfurther coupled with a corresponding one of multi-carrier modulators 214₁-214 _(M) (i.e., angular shift delay module 208 ₁ is coupled with1^(st) modulator 214 ₁, angular shift delay module 208 ₂ is coupled with2^(nd) modulator 214 ₂ etc.).

A signal X is provided to each of first delay modules 204 ₁-204 _(N).Signal X includes a plurality of symbols, each associated with the samesymbol period. Each of first delay modules 204 ₁-204 _(N) delays signalX by a respective first time delay T₁-T_(N). Each of delays 204 ₁-204_(N) produces a corresponding signal XT₁-XT_(N). Signals XT₁-XT_(N) aredelayed copies of signal X. In general, the number of delays correspondsto the number of beams and each copy is associated with a respectivebeam. Thus, each beam is associated with a respective time delay.Furthermore, the difference between the delays introduced by twoadjacent first delays (i.e., T_(n)−T_(n1)) should be as larger aspossible. Specifically this difference is determined to be larger thanthe inverse of the signal bandwidth (i.e., larger than a sample period)as follows:

$\begin{matrix}{{{T(n)} - {T\left( {n - 1} \right)}} > \frac{1}{BW}} & (1)\end{matrix}$

wherein BW is the bandwidth of the signal.

Each of first delay modules 204 ₁-204 _(N) provides the correspondingproduced signal thereof to beam pattern former 206. Beam pattern former206 adjusts the phase (e.g., multiplying by a complex weighting factor)of each of XT₁-XT_(N), and produces transmit signals XB₁-XB_(M).Transmit signals XB₁-XB_(M) correspond to the beam pattern (i.e., thenumber of beams, the corresponding direction of each beam and the widthof each beam). When transmitted, transmit signals XB₁-XB_(M) create thebeams, each beam having a corresponding spatial direction and respectivetime delay. In general beam pattern former 206 may be implemented as abeam forming matrix. For example, for a system with four beams and fourantennas the beam forming matrix may be an orthonormal rotation matrixW:

$\begin{matrix}{W = {\frac{1}{2}\begin{bmatrix}1 & 1 & 1 & 1 \\1 & j & {- j} & {- 1} \\1 & {- 1} & {- 1} & 1 \\1 & {- j} & j & {- 1}\end{bmatrix}}} & (2)\end{matrix}$

wherein j represents a phase shift of

$\frac{\pi}{2}.$

Beam pattern former 206 provides XB₁-XB_(M) to frequency dependent beamshifter 210. Beam pattern former 206 provides each of XB₁-XB_(M) to arespective one of angular shift delay modules 208 ₁-208 _(M) infrequency dependent beam shifter 210. Angular shift delay modules 208₁-208 _(M) delay each of XB₁-XB_(M) by a respective one of angular shiftdelays D₁-D_(M), producing delayed transmit signals XD₁-XD_(M). Ingeneral, the difference between two adjacent angular shift delays (i.e.,D_(n)-D_(n−1)) is determined to be on the order of the inverse of thebandwidth in use, as follows:

$\begin{matrix}{{D(n)} - {\left. {D\left( {n - 1} \right)} \right.\sim\frac{1}{BW}}} & (3)\end{matrix}$

Angular shift delay modules 208 ₁-208 _(M) introduce a phase shift tosignal respective of the sub-carrier frequency. Thus, an angular shiftis applied to each beam according to the sub-carrier frequency. Inparticular, to direct the beams in one carrier frequency toward adirection not covered by two adjacent beams, transmitted over adjacentsub-carrier frequencies, the difference between two adjacent angularshift delays (i.e., D_(n)-D_(n−1)) is determined to be:

$\begin{matrix}{{{D(n)} - {D\left( {n - 1} \right)}} \approx \frac{1}{M*{BW}}} & (4)\end{matrix}$

Thus, with reference back to FIGS. 2A and 2B, when beam 106 istransmitted at sub-carrier frequency F₂, beam 106 is directed toward thedirection indicated by arrow 126 (FIG. 2B) which is not covered byeither beam 106 or beam 108 when beams 106 and 108 are transmitted atsub-carrier F₁ (FIG. 2A).

Each of angular shift delay modules 208 ₁-208 _(M) provide delayedtransmit signals XD₁-XD_(M) to the corresponding modulator angular 214₁-214 _(M) thereof. Each of modulators 214 ₁-214 _(M) modulates therespective sub-carrier frequency thereof, with a respective one ofdelayed transmit signal XD₁-XD_(M). In general, the number ofsub-carrier frequencies is not equal to the number of signals. Thus, adelayed transmit signal may modulate a plurality of sub-carriers. Eachof modulators 214 ₁-214 _(M) of multi-carrier modulator 212 provides themodulated signals to front end interface 216. Front end interface 216performs operations such as up-conversion, filtering and the like, andtransmits the signals via antennas 218 ₁-218 _(M). In system 200, asmentioned above, angular shift delay modules 208 ₁-208 _(M) introduce arespective time delay D₁-D_(M), to each signal. Time delays D₁-D_(M)introduce phase shifts to each of the modulated signals. Since thesub-carrier frequencies, of each modulated signal is different, thephase shift introduced to each modulated signal will also be different.Thus, the direction of each beam will shift for each sub-carrierrelative to the other sub-carriers, according to the phase shiftintroduced to that sub-carrier. Thus referring back to FIGS. 2A, 2B and2C, beam 106 is directed at a direction indicated by arrow 114 atsub-carrier F₁ (FIG. 2A), at a direction indicated by arrow 126 atsub-carrier F₂ (FIG. 2B) and at a direction indicated by arrow 134 atsub-carrier F₃ (FIG. 2C). Additionally, beams 106, 108, 110 and 112maintain their relative positions there between.

It is noted that beam pattern former 206 described hereinabove is notfrequency or time dependent. However, beam pattern former 206 andfrequency dependent beam shifter 210 may be replaced with a frequencydependent beam former (i.e., each carrier frequency is associated with acorresponding beam pattern former). It is further noted thatmulti-carrier modulator 212 may be placed before transmit delay module202.

System 200, described hereinabove in conjunction with FIG. 3, timedelays signal X to create the spatial delay diversity with carrierfrequency dependent beam direction shifting. One exemplaryimplementation of system 200 (FIG. 3) is using N time-domain filters(i.e., as defined hereinabove—N is the number of beams). Reference isnow made to FIG. 4A, which is a schematic illustration of an exemplarysystem, generally referenced 250, for producing beam delay diversitywith frequency dependent beam direction shifting, constructed andoperative in accordance with a further embodiment of the disclosedtechnique. System 250 includes a transmit delay module 252, a beampattern former 256, a frequency dependent beam shifter 258, amulti-carrier modulator 262, a front end interface 266 and two antennas268 ₁ and 268 ₂. Transmit delay module 252 includes a single first delaymodule 254. Frequency dependent beam shifter 258 includes a singleangular shift delay module 260. Multi-carrier modulator 262 includesfirst modulator 264 ₁ and second modulator 264 ₂ each associated with arespective sub-carrier frequency. Beam pattern former 256 is coupledwith first delay module 254 and with angular shift delay module 260 andwith first modulator 264 ₁. Front end interface 266 is coupled with eachof antennas 268 ₁ and 268 ₂ and with each of first modulator 264 ₁ andsecond modulator 264 ₂. Second modulator 264 ₂ is further coupled withangular shift delay module 260. In system 250, beam pattern former 256is implemented as a two by two rotation matrix as follows:

$\begin{matrix}{W_{1} = {\frac{1}{\sqrt{2}}\begin{bmatrix}1 & 1 \\1 & {- 1}\end{bmatrix}}} & (5)\end{matrix}$

System 250 is an exemplary implementation of system 200 (FIG. 3) withtwo beams (i.e., N=2) where one beam does not exhibit a spatial delayand the other exhibits a spatial time delay of T₁. Furthermore, thedirection of the two beams is shifted in frequency according to thedelay introduced by angular shift delay module 260. Transmit delaymodule 252, beam pattern former 256 and frequency dependent beam shifter258 may all be implemented as two time-domain filters exhibiting thefollowing impulse response:

$\begin{matrix}{{h_{1}(t)} = {\frac{1}{\sqrt{2}}\left\lbrack {{\delta (t)} + {\delta \left( {t - T_{1}} \right)}} \right\rbrack}} & (6) \\{{h_{2}(t)} = {\frac{1}{\sqrt{2}}\left\lbrack {{\delta \left( {t - D_{1}} \right)} - {\delta\left( {t - T_{1} - D_{1}} \right\rbrack}} \right)}} & (7)\end{matrix}$

Reference is now made to FIG. 4B, which is a schematic illustration of asystem, generally referenced 280, for employing spatial delay diversitywith carrier frequency dependent beam direction shifting, constructedand operative in accordance with another embodiment of the disclosedtechnique. In system 280, the delay diversity and carrier frequencydependent beam direction shifting is implemented using two time-domainfilters exhibiting the impulse response of equations (6) and (7). System280 includes first time-domain filter 282, second time-domain filter284, multi-carrier modulator 286 and front end interface 290 andantennas 292 ₁ and 292 ₂. Multi-carrier modulator 286 includes a firstmodulator 288 ₁ and a second modulator 288 ₂ each associated with arespective sub-carrier frequency. First modulator 288 ₁ is coupled withfront end interface 288 and with first time-domain filter 282. Secondmodulator 288 ₂ is coupled with front end interface 288 and with secondtime-domain filter 282. Front end interface 290 is further coupled witheach of antennas 292 ₁ and 292 ₂.

First time-domain filter 282 is associated with the impulse response ofequation (6) and second time-domain filter 284 is associated with theimpulse response of equation (7). In FIG. 4B first time-domain filter282 and second time-domain filter 284 are depicted as the graphicalrepresentations of the respective impulse responses thereof.Accordingly, an arrow 294 represents δ(t) (i.e., a delta function withno delay) in equation (6) and an arrow 296 represents δ(t−T₁) (i.e., adelta function with a delay of T₁) in equation (6). An arrow 300represents δ(t−D₁) (i.e., a delta function with a time delay of D₁) inequation (7) and an arrow 302 represents δ(t−T₁−D₁) (i.e., a deltafunction with a time delay of T₁₊D₁) in equation (7). Arrows 298 and 304represent a time delay of T₁ (i.e., corresponding to first delay T₁ inFIG. 4A). Arrows 306 and 308 represent a time delay of D₁ (i.e.,corresponding to angular shift delay D₁ in FIG. 4A).

Signal X is provided to first time-domain filter 282 and to secondtime-domain filter 284. Signal X is convolved with the impulse responseof first time-domain filter 282 and with the impulse response of secondtime-domain filter 284. First time-domain filter 282 produces arespective signal corresponding to the sum of two copies of signal Xwith a delay of T₁ there between. Second time-domain filter 284 producesa respective signal corresponding to the sum of a copy of signal X andan inverted copy of signal X. The copy of signal X and the inverted copyof signal X corresponding to second time-domain filter 284 exhibit atime delay of T₁ there between and a time delay of D₁, relative to thecopies produced by first time-domain filter 282. Thus, first time-domainfilter 282 produces signal XD₁ and second time-domain filter 284produces signal XD₂. First time-domain filter 282 provides signal XD₁ tofirst modulator 288 ₁ and second time-domain filter 284 provides signalXD₂ to second modulator 288 ₂. First modulator 288 ₁ modulates therespective sub-carrier frequency thereof with signal XD₁. Secondmodulator 288 ₂ modulates the respective sub-carrier frequency thereofwith signal XD₂. First modulator 288 ₁ and second modulator 288 ₂provide the modulated signals to front end interface 290. Front endinterface 290 performs operations such as up-conversion, filtering andthe like and transmits the signals via antennas 292 ₁ and 292 ₂.

The system, according to the disclosed technique, may be adapted fortransmitting a plurality of signals. Thus, each signal is transmittedover a plurality of beams at time delays associated with these beams.Each beam is directed toward a different direction. Furthermore, thedirection of each beam shifts for each sub-carrier relative to the othersub-carriers.

Reference is now made to FIG. 5, which is a schematic illustration of asystem, generally reference 330, for employing spatial delay diversitywith frequency dependent beam direction shifting, constructed andoperative in accordance with a further embodiment of the disclosedtechnique. System 330 transmits according to a multi-carriertransmission scheme. In system 330, each of the two signals, X₁ and X₂,is transmitted over two carrier signals and two beams. Each of signalsX₁ and X₂ includes a plurality of symbols, each associated with the samesymbol period. System 330 includes a transmit delay module 332, a beampattern former 338, a frequency dependent beam shifter 340, amulti-carrier modulator 346, a front end interface 348 and a pluralityof antennas 352 ₁-352 ₄. In system 330, transmit delay module 332includes first delay modules 334 ₁ and 334 ₂ associated with signal X₁and first delay modules 336 ₁ and 336 ₂ associated with signal X₂.Frequency dependent beam shifter 340 includes angular shift delaymodules 344 ₁ and 344 ₂ associated with signal X₁ and angular shiftdelay modules 346 ₁ and 346 ₂ associated with signal X₂. Multi-carriermodulator 346 includes a first modulator 348 ₁, a second modulator 348 ₂a third modulator 348 ₃ and a fourth modulator 348 ₄, each associatedwith a respective sub-carrier frequency.

Beam pattern former 388 is coupled with each of first delay modules 334₁, 334 ₂, 336 ₁ and 336 ₂ and with each of angular shift delay modules342 ₁, 342 ₂, 344 ₁ and 344 ₂. Front end interface 350 is coupled witheach of antennas 352 ₁-352 ₄ and with each of modulators 348 ₁-348 ₄.Modulator 348 ₁ is further coupled with angular shift delay modules 342₁. Modulator 348 ₂ is further coupled with angular shift delay modules342 ₂. Modulator 348 ₃ is further coupled with angular shift delaymodules 344 ₁. Modulator 348 ₄ is further coupled with angular shiftdelay modules 344 ₂.

Signal X₁ is provided to each of first delay modules 334 ₁ and 334 ₂.Signal X₂ is provided to each of first delay modules 336 ₁ and 336 ₂.First delay modules 334 ₁ and 334 ₂ delay signal X₁ by correspondingfirst time delays T₁ and T₂. First delay modules 336 ₁ and 336 ₂ delaysignal X₂ by corresponding first time delays T₁ and T₂. First delaymodules 334 ₁ and 334 ₂ produce signals X₁T₁ and X₁T₂ respectively andprovide these signals to beam pattern former 338. First delay modules336 ₁ and 336 ₂ produce signals X₂T₁ and X₂T₂ respectively and providethese signals to beam pattern former 338. X₁T₁ and X₁T₂ are delayedcopies of signal X₁ and X₂T₁ and X₂T₂ are delayed copies of signal X₂.

Beam pattern former 338 adjusts (e.g., multiplying by a weightingfactor) each of X₁T₁, X₁T₂, X₂T₁ and X₂T₂ and produces transmit signalsXB₁-XB₄. Transmit signals XB₁-XB₄ correspond to a beam pattern (i.e.,the number of beams, the corresponding direction of each beam and thewidth of each beam). When transmit signals XB₁-XB₄ create the beams,each beam is created with its corresponding spatial direction and itsrespective time delay. Beam pattern former 338 produces transmit signalsX₁B₁ and X₁B₂ associated with signal X₁ and transmit signals X₂B₃ andX₂B₄ associated with signal X₂. Beam pattern former 338 providestransmit signal to X₁B₁ to angular shift delay module 342 ₁ and signalX₁F₂ to angular shift delay module 342 ₂. Beam pattern former 338further provides signal to X₂B₃ to angular shift delay module 344 ₁ andsignal X₂B₄ to angular shift delay module 344 ₂. Angular shift delaymodule 342 ₁ delays signal X₁B₁ by a delay D₁ and produce a delayedtransmit signal X₁D₁. Angular shift delay module 342 ₂ delays signalX₁B₂ by a time delay D₂ and produce a delayed transmit signal X₁D₂.Angular shift delay module 344 ₁ delays signal X₂B₃ by a time delay D₀and produce a delayed transmit signal X₂D₁. Angular shift delay module344 ₂ delays signal X₂B₄ by a delay D₁ and produce a delayed transmitsignal X₂D₂. Angular shift delay 342 ₁ provides the delayed transmitsignal X₁D₁ to modulator 348 ₁. Angular shift delay 342 ₂ provides thedelayed transmit signal X₁D₂ to modulator 348 ₂. Angular shift delay 344₁ provides the delayed transmit signal X₂D₁ to modulator 348 ₃. Angularshift delay 344 ₂ provides the delayed transmit signal X₂D₂ to modulator348 ₄. Modulator 348 ₁ modulates signal X₁D₁ by the respective carrierfrequency thereof. Modulator 348 ₂ modulates signal X₁D₂ by therespective carrier frequency thereof. Modulator 348 ₃ modulates signalX₂D₁ by the respective carrier frequency thereof. Modulator 348 ₄modulates signal X₂D₂ by the respective carrier frequency thereof. Eachof modulators 348 ₁-348 ₄ provides the modulated signal thereof to frontend interface 350. Front end interface 350 performs operations such asup-conversion, filtering and the like and transmits the signals viaantennas 352 ₁-352 ₄.

As mentioned above, the disclosed technique can be implemented in thefrequency domain, by having the phases of the signals shifted.Accordingly, the time-delays are implemented as multiplications by acomplex exponential. Reference is now made to FIG. 6, which is aschematic illustration of a system, generally reference 380, foremploying spatial delay diversity and frequency dependent beam directionshifting, constructed and operative in accordance with anotherembodiment of the disclosed technique. System 380 is implemented in thefrequency domain. System 380 includes a transmit delay module 382, abeam pattern former 386, a frequency dependent beam shifter 390, amulti-carrier modulator 392, a front end interface 396 and a pluralityof antennas 398 ₁-398 _(M). Transmit delay module 382 includes a firstphase shifter 384. Multi-carrier modulator 392 includes a plurality ofmodulators 394 ₁-394 _(M) each associated with a respective sub-carrierfrequency. Frequency dependent beam shifter 390 includes a second phaseshifter 388. Beam pattern former 386 is coupled with first phase shifter384 and with second phase shifter 388. Front end interface 396 iscoupled with each of antennas 398 ₁-398 _(M) and with each of modulators394 ₁-394 _(M). Second phase shifter 388 is further coupled with each ofmodulators 394 ₁-394 _(M).

When system 380 transmits four beams over four frequencies, first phaseshifter 384 is, for example, a matrix of the form:

$\begin{matrix}{C_{1} = \begin{bmatrix}1 & 0 & 0 & 0 \\0 & ^{{- {j\omega}}\; T_{1}} & 0 & 0 \\0 & 0 & ^{{- {j\omega}}\; T_{2}} & 0 \\0 & 0 & 0 & ^{{- {j\omega}}\; T_{3}}\end{bmatrix}} & (7)\end{matrix}$

wherein ω denotes frequency T_(n) as defined above.

When system 380 transmits four beams over four frequencies, second phaseshifter 388 is, for example, a matrix of the form:

$\begin{matrix}{C_{2} = \begin{bmatrix}1 & 0 & 0 & 0 \\0 & ^{{- {j\omega}}\; D_{1}} & 0 & 0 \\0 & 0 & ^{{- {j\omega}}\; D_{2}} & 0 \\0 & 0 & 0 & ^{{- {j\omega}}\; D_{3}}\end{bmatrix}} & (8)\end{matrix}$

wherein D_(n) is as defined above. Beams former 386 is similar to beampattern former 206 described hereinabove in conjunction with FIG. 3.

A signal X is provided to first phase shifter 384. Signal X includes aplurality of symbols, each associated with the same symbol period. Firstphase shifter 384 shifts the phase of signal X by an angle respective ofthe delay of each beam. First phase shifter 384 produces signalsXT₁-XT_(N) and provides these signals to beam pattern former 386.XT₁-XT_(N) are phase shifted copies of signal X.

Beam pattern former 386 adjusts (e.g., multiplying by a weightingfactor) each of XT₁-XT_(N) and produces transmit signals XB₁-XB_(M).Transmit signals XB₁-XB_(M) correspond to a beam pattern (i.e., thenumber of beams, the corresponding direction of each beam and the widthof each beam). When transmitted, transmit signals XB₁-XB_(M) create thebeams, each beam having a corresponding spatial direction and arespective time delay. Beam pattern former 386 produces transmit signalsXB₁-XB_(M) and provides these signals to second phase shifter 388.Second phase shifter 388 shifts the phase of each of transmit signalsXB₁-XB_(M) and produces phase shifted transmit signals XD₁-XD_(M).Second phase shifter 388 provides phase shifted transmit signalsXD₁-XD_(M) to multi-carrier modulator 392. Second phase shifter 388provides phase shifted transmit signals XD₁-XD_(M) to a correspondingone of modulators 394 ₁-394 _(M) (i.e., Signal XD₁ is provided tomodulator 394 ₁, signal XD₂ is provided to modulator 394 ₂ etc.) Each ofmodulator 394 ₁-394 _(M) modulates each of the sub-carriers according tothe corresponding signal provided thereto. Each of modulators 394 ₁-394_(M) provides the modulated signal thereof to front end interface 396.Front end interface 396 performs operations such as up-conversion,filtering and the like and transmits the signals via antennas 398 ₁-398_(M).

As mentioned above, the direction of each beam is essentially confinedin a determined angular sector. Thus the transmitted power isconcentrated only toward that determined angular sector, resulting in atransmit power gain. Reference is now made to FIG. 7 which is aschematic illustration of a system, generally referenced 420, forproducing transmit delay diversity with frequency dependent beamdirection shifting, operative in accordance with a further embodiment ofthe disclosed technique. In system 420, the direction corresponding toeach beam is essentially confined in a determined angular sector. System420 includes a transmitter 422 coupled with antennas 424 ₁, 424 ₂ and424 _(M). In FIG. 7 the direction of each of beams 426, 428 and 430 isessentially confined in an angular sector defined by a line 432 and aline 434. Consequently, the transmit power is concentrated only towardthat angular sector resulting in a transmit power gain. Transmitter 422produces the angular sector confined beams by employing more antennasthan beams. In other words, the beam pattern former produces a largernumber of transmit signals than the number of copies of the signal X.Accordingly, the beam forming matrix is non-square. For example, forproducing three beams, limited within an angular sector, four antennasare used and the beam forming matrix is:

$\begin{matrix}{W = {\sqrt{\frac{4}{3}}\begin{bmatrix}1 & 1 & 1 \\1 & j & {- j} \\1 & {- 1} & {- 1} \\1 & {- j} & j\end{bmatrix}}} & (9)\end{matrix}$

wherein j represents a phase shift of

$\frac{\pi}{2}.$

The frequency shift delay is determined to be:

$\begin{matrix}{D_{n} \approx \frac{\left( {n - 1} \right){\pi/4}}{{BW}/2}} & (10)\end{matrix}$

It is noted that each of the systems described hereinabove may includeadditional components such as Forward Error Correction (FEC) encoder(e.g., convolutional encoder Reed-Solomon encoder and the like) and abase band modulator (e.g., Quadrature Amplitude Modulator—QAM, PhaseShift Keying—PSK modulator and the like) placed before the transmitdelay module.

Reference is now made to FIG. 8, which is a schematic illustration of amethod for producing beam delay diversity with frequency dependent beamdirection shifting, operative in accordance with another embodiment ofthe disclosed technique. In procedure 500, a beam forming matrix isdetermined. The beam forming matrix corresponds to a determined beampattern. The method proceeds to procedures 502 and 504.

In procedure 502, a transmit delay is determined for each beam in thebeam pattern. The method proceeds to Procedure 506.

In procedure 504, copies of the signal are produced. The signal is to betransmitted over a plurality of carrier frequencies. The number ofcopies corresponds to the number of beams in the beam pattern.

In procedure 506, each copy of the signal is delayed by a correspondingtransmit delay. With reference to FIG. 3, transmit delay module 202delays each copy of signal X by a corresponding transmit delay T₁-T_(N)associated with first delay modules 204 ₁-204 _(N). In procedure 508,the beam forming matrix is applied to the delayed copies of the signal,thereby producing transmit signals corresponding to the beam pattern. Itis noted that each transmit signal is associated with at least onecarrier frequency. With reference to FIG. 3, beam pattern former 206produces transmit signals XB₁-XB_(M). Transmit signals XB₁-XB_(M)correspond to the beam pattern. The method proceeds to procedure 512.

In procedure 510, an angular shift delay is determined for each transmitsignal. It is noted that procedure 510 is independent from any of thepreceding procedures.

In procedure 512, each of the transmit signals is delayed by arespective angular shift delay. Thereby an angular shift is applied toeach beam in the beam pattern. The angular shift of each beamcorresponds to each carrier frequency. With reference to FIG. 3,frequency dependent beam shifter 210 delays each of transmit signalsXB₁-XB_(M) by a respective angular shift delays D₁-D_(M) associated withangular shift delay modules 208 ₁-208 _(M).

In procedure 514, each transmit signal is transmitted via acorresponding antenna. With reference to FIG. 3, angular shift delaymodules 208 ₁-208 _(M) provide delayed transmit signals XD₁-XD_(M) tomulti-carrier modulator 212. Multi-carrier modulator 212 modulates eachof the sub-carriers with a respective delayed transmit signalXD₁-XD_(M). Front end interface 214 transmits the signals via antennas216 ₁-216 _(M).

The multi-carrier modulator described hereinabove in conjunction witheach of FIGS. 3, 4A, 4B, 5 and 6, is placed after the frequencydependent beam shifter. It is, However, noted that, as described abovein conjunction with FIG. 3, the multi-carrier modulator may be placedbefore the transmit delay module. Additionally, it is noted that each ofthe delays described hereinabove may be implemented as a digital delayor an analog delay. Furthermore, the delays may be cyclic delays. Acyclic delay is achieved by cyclically shifting a symbol within thesymbol period. Thus, the delay spread of the received signal remainsunchanged. It is further noted that the system according to thedisclosed technique, described hereinabove is especially effective whenthe antennas in the antenna array are correlated (i.e., the spacingbetween the antennas is small or a Line Of Sight (LOS) exists betweenthe transmitter and receiver. However, the system according to thedisclosed technique may employ antenna arrays, wherein the spacingbetween two adjacent antennas is substantially large and no LOS existsbetween the transmitter and receiver.

It will be appreciated by persons skilled in the art that the disclosedtechnique is not limited to what has been particularly shown anddescribed hereinabove. Rather the scope of the disclosed technique isdefined only by the claims, which follow.

1. In a wireless transmission system including a transmit delay module,delaying each of at least one copy of a signal by a respective delay,the signal being transmitted over a plurality of carrier frequencies andover at least one beam, the at least one beam exhibiting a beam pattern,the number of copies of the signal corresponding to the number of beams,each said at least one copy being associated with a respective one ofsaid at least one beam, said system further including a beam patternformer, coupled with said transmit delay module, producing a pluralityof transmit signals corresponding to the beam pattern, a frequencydependent beam shifter, coupled with the beam pattern former, delayingeach of the at least one transmit signal by a respective angular shiftdelay, thereby applying an angular shift to each of the at least onebeam, the angular shift of each of the at least one beam correspondingto at least a respective one of the carrier frequencies, wherein eachbeam is transmitted with a delay corresponding to the transmit delay ofeach said copy.
 2. The device according to claim 1, wherein saidfrequency dependent beam shifter module includes a plurality of angularshift delay modules, each of said angular shift delay modules beingcoupled with said beam pattern former, each of said angular shift delaymodules, delaying a respective transmit signal by a respective timedelay.
 3. The device according to claims 2, wherein each said respectivetime delays is on the order of a signal sample period associated withsaid signal.
 4. The device according to claim 2, wherein each of saidangular shift delay modules cyclically delays said respective transmitsignal.
 5. The device according to claim 1, wherein said frequencydependent beam shifter includes a phase shifter, coupled with said beampattern former, said phase shifter shifting the phase of each of said atleast one transmit signal.
 6. A wireless transmission system comprising:a transmit delay module, delaying each of at least one copy of a signalby a respective delay, said signal being transmitted over a plurality ofcarrier frequencies and over at least one beam, said at least one beamexhibiting a beam pattern, the number of copies of said signalcorresponding to the number of beams, each said at least one copy beingassociated with a respective one of said at least one beam; a beampattern former, coupled with said transmit delay module, producing aplurality of transmit signals corresponding to said beam pattern; and afrequency dependent beam shifter, coupled with said beam pattern former,delaying each of said transmit signals by a respective angular shiftdelay, thereby applying an angular shift to each of said at least onebeam, the angular shift of each of said at least one beam correspondingto at least a respective one of said carrier frequencies, wherein eachbeam is transmitted with a delay corresponding to the transmit delay ofeach said copy.
 7. The system according to claim 6, wherein saidtransmit delay module includes a plurality of first delay modules, eachof said first delay modules is coupled with said beam pattern former,each of said first delay modules, delays a respective copy of saidsignal by a respective first time delay.
 8. The system according toclaim 7, wherein each said respective first time delays is larger than asignal sample period associated with said signal.
 9. The systemaccording to claim 7, wherein each of said first delay modulescyclically delays each said respective copy of signal.
 10. The systemaccording to claim 6, wherein said transmit delay module includes aphase shifter, coupled with said beam pattern former, said phase shiftershifting the phase of each of said copies of said signal by a respectivephase shift.
 11. The system according to claim 6, wherein said beampattern former adjusts each of the delayed copies of said signal. 12.The system according to claim 11, wherein said beam pattern formerincludes a beam forming matrix.
 13. The system according to claims 12,wherein said beam forming matrix is an orthonormal rotation matrix. 14.The system according to claim 12, wherein said beam pattern former formssaid beam pattern by multiplying said delayed copies of said signal bysaid beam forming matrix.
 15. The system according to claim 6, whereinthe directions of said at least one beam of said beam pattern isessentially confined in an angular sector.
 16. The system according toclaim 15, wherein said beam pattern former produces said beam pattern,which exhibits said at least one beam essentially confined in saidangular sector, by producing a larger number of said transmit signalsthen said copies of said signal.
 17. The device according to claim 6,wherein said frequency dependent beam shifter includes a plurality ofangular shift delay modules, each of said angular shift delay modulesbeing coupled with said beam pattern former, each of said angular shiftdelay modules, delaying a respective one of said transmit signals by arespective angular shift delay.
 18. The system according to claims 17,wherein each of said respective time delay is on the order of saidsignal sample period.
 19. The system according to claims 17, whereineach of said angular shift delay modules cyclically delays saidrespective one transmit signal.
 20. The device according to claim 6,wherein said frequency dependent beam shifter includes a phase shifter,coupled with said beam pattern former, said phase shifter shifting thephase of each of said at least one transmit signal.
 21. The systemaccording to claim 6, wherein said transmit delay module, said beampattern former and said frequency dependent beam shifter are formed of aplurality of time-domain filters, the number of said time-domain filterscorresponds to the number of beams in said beam pattern, the impulseresponse of said time-domain filters corresponds to said beam pattern,each said impulse response of each said time-domain filter is delayed bya corresponding angular shift delay.
 22. Method for transmitting asignal over a plurality of carrier frequencies and a plurality of beams,the beams exhibiting a beam pattern, the method comprising theprocedures of: delaying each copy of the signal by a correspondingtransmit delay; applying a beam forming matrix to the delayed copies ofthe signal, thereby producing transmit signals corresponding to the beampattern, each transmit signal being associated with at least one carrierfrequency; and delaying each of said transmit signals by a respectiveangular shift delay, thereby applying an angular shift to each beam inthe beam pattern, said angular shift of each beam corresponding to eachcarrier frequency.
 23. The method according to claim 22, furtherincluding the preliminary procedures of: producing copies of the signalto be transmitted over said plurality of carrier frequencies, the numberof said copies corresponding to the number of said beams in a beampattern; and determining a transmit delay for each of said beams in saidbeam pattern.
 24. The method according to claim 23, further including,prior to said procedures of producing copies and determining a transmitdelay, the procedure of determining said beam forming matrixcorresponding to said beam pattern.
 25. The method according to claim22, further including, prior to said procedure of delaying each of saidtransmit signals by a respective angular shift delay, the procedure ofdetermining an angular shift delay for each of said transmit signals.26. The method according to claim 22, further including the procedure oftransmitting each of said transmit signal via a corresponding antenna,after said procedure of delaying each of said transmit signals by arespective angular shift delay.